In a wireless LAN (Local Area Network) or a wireless PAN (Personal Area Network) using UWB (Ultra Wide Band), a plurality of PHY modes are defined, and an optimum PHY mode is selected according to the current communication quality (transmission line status) to perform control so as to generally provide a packet error rate of 10% or less (in general, referred to as an “adaptive modulation scheme” or the like).
For example, in IEEE 802.11a, eight PHY modes of 6 Mbps to 54 Mbps are defined. Further, in a WiMedia PHY MB-OFDM (Multi Band-Orthogonal Frequency Division Multiplexing) scheme, which is a UWB scheme used for Wireless USB (Universal Serial Bus), eight PHY modes of 53.3 Mbps to 480 Mbps are defined. In such a system in which a plurality of PHY modes are defined, a method of selecting an optimum PHY mode according to the communication quality has hitherto been proposed.
Hitherto, in a case where an adaptive modulation scheme is performed, a method has been adopted in which a pilot signal is monitored to estimate a state of a transmission line (see Patent Document 1) or in which a packet error rate or packet arrival delay with respect to a certain number of packets to be sent is monitored and is improved by switching a PHY mode to that of a one step lower rate in a case where the error rate or delay exceeds a predetermined value (see Patent Document 2).
In connection to this, substantially no consideration has been given regarding which method to use to monitor a transmission line state or packet error rate, and a method has merely been examined in which in order to reduce variations in monitoring operations, the number of monitoring operations is increased to some extent to increase the average number. If packet errors were caused by a time-independent interference signal or gauss noise, adaptive modulation based on this monitoring method would be performed without difficulty. In a case where time-dependent interference exists, however, a problem occurs.
For example, in an MB-OFDM scheme adopted in WUSB (Wireless USB), time-dependent interference exists. A description of this interference will be first given followed by what problem occurs if an adaptive modulation scheme using a method of the related art is applied under the existence of time-dependent interference.
In a WiMedia PHY MB-OFDM scheme, as in FIG. 6, seven “channels” are defined. In this regard, a “channel” in the eMB-OFDM scheme does not refer to a standard frequency channel but is used to mean that hopping patterns are different within an identical frequency band. That is, even different channels would interfere with each other.
As shown in FIG. 7, focusing on a channel of TFC:1 and a channel of TFC:2 as an example, signal collision occurs in a sub-band 1 (Subband 1) once every three symbols. For example, in FIG. 8, in a case where communication apparatuses #1 and #2 communicate with each other over TFC:1 and communication apparatuses #3 and #4 communicate with each other over TFC:2, the above collision occurs in the communication apparatus #2. Depending on the positional relationship between communication apparatuses or the transmission power, in a case where radio waves that reach the communication apparatus #2 from the communication apparatuses #1 and #3 have substantially the same signal intensity, the communication apparatus #2 is prevented from correctly receiving signals of Subband 1.
In the MB-OFDM scheme, as shown in FIG. 9, eight PHY modes exist, where Time Domain Spreading (TDS) is not implemented for the 320 Mbps, 400 Mbps, and 480 Mbps modes and, as described above, incorrect reception on Subband 1 would cause insufficient error correction, resulting in a packet error. On the other hand, Time Domain Spreading is implemented for the modes of 200 Mbps or less, and data having the same content is sent using two consecutive symbols. Thus, incorrect reception at Subband 1 would not cause a packet error if reception at next, Subband 2 can be correctly performed.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-250116    Patent Document 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2004-519894